Recently, in the field of printing plate making and medical treatments, effluents produced in wet process of image forming materials produce problems in working property, and reduction of the effluents is strongly desired in terms of environment protection and saving space. In response thereto, techniques of photothermal materials capable of forming sharp and clear black images on thermal development are needed. Such a technique is described in D. Morgan, "Dry Silver Photographic Materials" in Handbook of Imaging Materials, published by Marcel Dekker Inc. (1991) at page 48. The photographic materials described therein are developed at a temperature of 80.degree. C. or higher and called a thermally developable photosensitive material.
However, it has been known that such a type of thermally developable photosensitive materials easily produce marked fog when allowed to stand in an atmosphere at a high humidity for a long period of time, leading to unfavorable results. In almost of the thermally developable photosensitive materials, a coating solution using organic solvents such as toluene, methyl ethyl ketone or methanol is coated to form a image forming layer. This is because polyvinyl acetal such as polyvinyl butyral used as a binder is soluble only in organic solvents. However, the use of an organic solvent is disadvantageous in terms of bad influences on human body in the manufacturing process and costs for recovering the solvent.
JP-A 49-562626, 50-151138, 53-116144, 58-28737 and 60-61747 (hereinafter, the term, JP-A means an unexamined, published Japanese Patent Application) describe a technique for forming the image forming layer by using a coating solution containing an aqueous solvent, in which aqueous soluble binders such as gelatin, polyvinyl alcohol and polyvinyl acetal are employed. Further, JP-A 10-73899 and 10-73901 describes a image forming layer containing a binder such as an aqueous-dispersible acryl type polymer or aqueous dispersible-dispersible polyvinyl acetal. The use of such a binder enables to form the image forming layer by using an aqueous solvent type coating solution, leading to merits in environment and cost. However, in cases where the image forming layer is formed using an aqueous coating solution, problems of fogging caused upon storage over a long period of time in atmosphere at high humidity become prominent. Further, it is desired to make the image forming layer thinner.